Normally when using AE's DB Client Copy it is able to complete the copy in an minutes/hours for very large clients. If Client Copy is taking longer than that (especially more than a day or more) than there are some things to consider.

For example, it would not be uncommon for the Client Copy to take 1+ days in the following circumstances.

If you are attempting to copy all statistics and Reports to the new client, this may increase the amount of time it takes to complete the copy depending on the amount of statistics and on your database performance.

The amount of versioning objects may also impact the time it takes depending on how you handle versions.

Resolution:

If you absolutely need to copy a large amount of data, there are some things that you should consider:

Pay attention to the CommitCnt Parameter in the ucybdbcc.ini and do not increase its value.

Try to run the Client Copy during a time frame where the workload is lower on the Automation Engine.